then
Implies that the action follows something else. One presumes that something was a catalyst for the action, but no guarantees are made by the statement.

i
We'll forego the exhaustive and pedantic argument concerning capitalization of "I" and appropriateness thereof in the interest of time, and glean only that the action being referred to is perpetrated by the speaker.

poked
This verb set in the past tense suggests that the action has already taken place - a notion supported by the initial context-setting "then". Poking can be some of the most enjoyable and entertaining activity known to humankind. It remains to be seen whether this instance will be pleasant or otherwise.

him
Clearly, the poking was done to a male of some variety. By peeking ahead (I know, I know. It's wrong. I'm sorry.) we can see that although species has yet to be determined, (and, at 57% of the way through the phrase, it is doubtful such information will reveal itself in time for consideration) it is probably safe to assume that the pokee is human (or porcine, the only other animal that sunburns (so Emeril Lagasse tells me)) and is therefore a boy or man (or a piglet or pig. Mmmmm...bacon).

in
A simple word like this might seem innocuous, but its inclusion in this knotty phrase may prove challenging to decode. For now, let's leave it alone until we've parsed out the rest of the players in our little seven-word play.

his
Here we see that the next item belongs to the pokee himself. I know I'm all a'quiver to see how this ends.

sunburn
At last the crux of all the poking, people, and possessiveness. A sunburn. Now we can evaluate the merits of "in".

in (again)
This is a tricky one, for in the context of then i poked him in his sunburn "in" does not strictly indicate "inside of", but rather denotates a region (of sunburned skin, I must conclude) within the borders of which said poking occurred.

When properly evaluated, we can clearly see that the speaker has prodded the sunburned skin of a male human. This tends to produce serious grousing and occasional fainting in pain, depending on severity of sunburn and poke.

Accomplished provocateurs know that the real money's in slapping sunburns. The sting of a solid swat coupled with the agony of broiled flesh makes for some explosive reactions indeed. Some slappers have been known to survive a really choice whacking of their reddened target, but most find that breathing is difficult when their lungs have been removed and tied in a bow. Deliver any pokes and/or slaps to sunburn victims with caution, and preferrably, a long-handled flyswatter. Those added seconds of escape time can make a big difference.